# Homework Help: An Electron Released from Rest

1. Apr 19, 2007

### Boozehound

An electron is released from rest at the negative plate of a parallel plate capacitor. The charge per unit area on each plate is σ = 1.69E-7 C/m2, and the plates are separated by a distance of 1.72E-2 m. How fast is the electron moving just before it reaches the positive plate?

i started by using the equation E=σ/ϵ. so i got E=1.9096E4N/C. then i multiplied that by one coulomb to get force. so F=3.05536E-15N. i then took newtons seconds law (F=ma) and found acceleration. for the mass of the electron i used 9.109E-31kg. so i was left with a=3.35E15m/s^2. i need to find out velocity, and so i looked for kinematic equations and i cant find one with all the variables that i have. it seems like everyone i try to use is missing 2 variables. am i overlooking something? please point me in the right direction. thanks!

2. Apr 19, 2007

### hage567

You can use this equation again:

$$v^2 = v_o^2 + 2ax$$

You have all of the values you need. Which ones are you unsure of?

3. Apr 19, 2007

### denverdoc

I would try to equate difference in PE's with KE and forget kinematics entirely.

(edit: pretty much end up with Hages eqn either way)

4. Apr 19, 2007

### Dick

As long as you've computed the acceleration, if you really want to use kinematics, use d=(1/2)*a*t^2 to compute the time and v=a*t to compute the final velocity. You have a superabundance of choices of how to complete the problem.

5. Apr 19, 2007

### Boozehound

in response to hage567 i dont think that i have the initial velocity. thanks to all three of you.

6. Apr 19, 2007

### Dick

"An electron is released from rest". Think again.